1. Field of the Invention
The present invention relates to improvements of a radio-frequency receiver device provided with a plurality of receiver antenna elements arranged to receive a signal transmitted from a desired communication object.
2. Description of the Related Art
There is known a radio-frequency identification system (an RFID system) including small-sized radio-frequency tags (transponders) storing predetermined information, and a radio-frequency tag communication device (interrogator) arranged to read desired information from the radio-frequency tags in a non-contact fashion. The radio-frequency communication device of this RFID system is capable of reading out the information from the radio-frequency tags, by radio communication with the radio-frequency tags, even where the radio-frequency tags are soiled or located at invisible places. For this reason, the RFID system is expected to be used in various fields such as management and inspection of commodities.
Such a radio-frequency tag communication device includes a radio-frequency receiver device which is provided with a plurality of receiver antenna elements to receive signals transmitted from communication objects in the form of radio-frequency tags and which is arranged to control the directivity of the receiver antenna elements for communication with the desired radio-frequency tag. JP-2003-283411A discloses an example of such a radio-frequency receiver device, and JP-2002-280945A discloses an example of control of the directivity of the receiver antenna elements. See paragraphs [0002]-[0035] and FIG. 3 of JP-2002-280945A. According to the techniques disclosed in these publications, the radio-frequency receiver device is provided with a receiver antenna device in the form of an array antenna device having a plurality of receiver antenna elements, and the received signals received by these receiver antenna elements are subjected to an adaptive array antenna control according to controlled weights given to the respective received signals. The received signals multiplied by the weights are combined together into a composite signal, which is demodulated for reading the information transmitted from the desired radio-frequency tag. The weights are updated so as to minimize an error between the modulated signal and a reference signal, for thereby maximize the sensitivity of reception of the received signals by the receiver antenna elements from the desired communication object, whereby the directivity of the array antenna device is suitably controlled for adequate radio communication between the radio-frequency communication device and the desired radio-frequency tag.
There is also known a phased array antenna control as another type of directivity control of the plurality of receiver antenna elements of the radio-frequency receiver device. According to the phased array antenna control, the directivity of reception of the receiver antenna elements is maximized in a selected one direction, which is changed from time to time to accurately detect the position or direction of the desired communication object or radio-frequency tag.
However, the conventional adaptive array antenna (AAA) processing requires a large volume of calculation of the weights to be given to the received signals received by the respective receiver antenna elements, according to a suitable adaptive processing algorithm, and therefore undesirably takes a considerable time for the large volume of calculation of the weights.
The radio-frequency communication system generally has an operating environment in which there are various electric wave hindrance or interference objects between the radio-frequency transmitter and receiver devices, so that the receiver antenna device of the radio-frequency receiver device provided to receive a desired reply wave directly from the radio-frequency transmitter device may also receive reflected waves which are components of the reply wave that are reflected by the electric wave hindrance or interference objects. The conventional adaptive array antenna control has a risk of controlling the directivity of reception of the array antenna device with respect to the reflected waves as well as the desired reply wave, so that the sensitivity of reception of the desired reply wave from the radio-frequency transmitter device may be deteriorated. On the other hand, the conventional phased array antenna control has a risk of erroneous determination or recognition that the direction in which the reflected waves are received is the direction in which the radio-frequency transmitter device is located. This risk of erroneous determination is high where a difference between the amplitudes of the desired replay wave and the reflected waves that have been received by the receiver antenna device is not so large. Accordingly, the conventional phased array antenna control does not assure efficient or rapid determination of the direction in which the radio-frequency transmitter device is located. Thus, there has been a need of developing a radio-frequency receiver device capable of efficient control of the directivity of the receiver antenna elements.